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Because the four piezoresistive elements comprising a Wheatstonebridge are formed simultaneously, their teiperature coefficients areclosely matched.Moreover, the silicon elements fonmd on a silicondiaphragm result in a minimum of stress due to thermal exansion mismatchof materials.The small sizes obtainable with this class of devices are possiblebecause of the microlithographic process employed.7his process is similarto that used in the manufacture of silicon integrated circuits.7he desiredpattern is first produced at 100x scale and then precisely reduced andstepped and repeated to form a number of images on a glass mask.Theseimages are oriented along crystallographic axes transferred to the silionrwafer by the technique of photoresist imaging, oxide fonition, and chemicaletching.7he result is that these devices can be batch fabricated, withtypically 50 to 100 devices mnufactured simultaneously on a 5-cm siliconwafer.The resulting sensors are thus nearly identical in their perfor-mance characteristics and are virtually operational pressure traonelements as diced from the wafer, requiring only suitable packaging, the1ualcxmpensation, and calibration to become a finished, high performaneinstrument.Of particular relevance to the class of transducers used inblast and shock neasurements is the ability to fabricate a very small activearea of typically less than 1 mu in diameter for the HKS-375 series pressuretransducers.The resonant frequency for a clamped-edge diaphragm is given byfn-9.66(1)ll

E is Young's modulus of elasticityT is the thicknsR is the disc's active radiusp is the density of the disc mterialMen silicon is used as the diaphrag material, the density (p) isabout a fourth that of the value for most metals.ant frequency of a silicoOonsequently, the reso-disc used as a diaphragm isrmally twice thatof a metal diaphragm of identical dimensions.Qmtining the constants in (1) allows fto be expressed asf KTfn whreKI12(2)is a constant.1 m radial stresses at the center of the diaram are given byR22T2(3whereK2 is a constantP is the pressure12*

-.-. .wers have the sm pressure range and are designedandthu a omprabe otuneto have the same radial stress, S., and thus a caparable output underassum that the tranwsthese conditions. The relationship between the thickness and the radius ofthe element isT2K 2 PR2(4)orT KRrcwhere K3 is a constant.Substituting for T in equation (2), the relationship describing thenatural resonant frequency isK3Rfn K, - K4R2 R(5)R' Rwhere K4 K I K 3 .This indicates that the natural frequency for tasrof the samepressure range and t3 same radial stresses in the center is inerselyproportional to the clamped or active radius.Thus, miniaturization plus the use of silicn for diaphragm materialprovides a susttial increase in the natural frequency.*For exmiple, goingfrom a 2.5-cmidiameter metal diaphragm pressure transducer with 2.0-euactive diameter to a 3.2-rm diameter with 2.0 -m active diameter silicndiaphragm results in an increase of 10 x 2 - 20 times in natural frequency;and going to a pressure transducer with an 0.75-mn diameter and an activediamterof 0.25 mnresults in an increase of 2 x 80 -160 tims innatural frequency.13 .-:.* ,.',,-j

The iuportance of the mrked increase in natural freumncy is theextension of the frequency response and the ability of the pressure trans-Aer to respond to the very rapid rise time phemnatered in highexplosive blast testing.2.IMTATI1N CW DIAPHRAGK APPI4IThe diaphragm approach, however, begins to show serious limitationsabove the range of 207 I4Pa7his is because, as the diameter-to-thidmessratio of the disc becomes sufficiently siall, undesirable shear stressesbecm a significant part of the total stress situation at the undersurfaceof the disc when loaded.7Iese shear forces, in general, subtract from thebending stresses which represent the nonml mode of operation of the device,and tend to create an acceptably high nonlinear response.Wn the diaphragm thickness-to-diameter ratio for a clais-edgecircular disc under norml pressure loading is about 0.1 or less, the theoryof sall dispats based on pure bending effects is quite accurate forpractical calculations.The pertinent results for deflection (D) and radialbending stress (a) as a function of radial distance (r) are as follows:D(r) Do(r) - ao-k 'IA&Y0-3R-1(OKntrin0prenaion)(V is Poisson's ratio)(l-1 ) P R4E T'CFON14(1 (6)

Do and oo are the deflection and radial stress at the center of thediaphragm, respectively.For thick plates, however, the effect of shearing stresses beo1he deflection D(r) due to oabined shearing of the middlesignificant.surface and bending of the plate is given byD(r) Do[(D(o) Do[ y][2 r(7)T1(8)AT( THEACENERAnd the effect of shearing on the bending stress is to translate theshape of the radial bending stress vs. radial distance to more negativevalues, namely(3 ,)(r)I (r) o'he aditionul tenn2V2-i(9)()1(9)in the stress equation indicates that whenthe thickness is about one-third the diameter, the edge strain becumes zeroand the center strain becomes more negative.As a result of this sheareffect, the total net stresses at the gage elements are reduced and the output of the transducer is correspondingly reduced.To achieve a given output,higher stresses nust be employed resulting in a more nonlinear response.Thus, the well-behaved linear response of a flat diaphragm is lost as thethickness-to-diameter ratio is increased beyond 0.1.3.IAD CYLINER APPRAOHa.Introduction--The above considerations dictate that for nmeasurmntof blast pressures above 2 kbar a new cxceptual approach is needed.concept developed in this effort is shown in Figure 3.15The

RANSVERSELY ACTIVEPIEZORESISTIVEELEMENT4 10ooc 110 SILICON LOADDISCINACTIVE LOADLOAD SENSORDISCfCYLINDERSILICON SUPPORTDISCMOUNTSUBASSEMBLYSENSORFIGURE 3. LOAD CYLINDER TYPE INTEGRATED SENSORPRESSURE TRANSDUCER16.,,,

Tesensing elemnt or silicon load cylinder is oaqxsed of an integratedsensor load disc and a silicon support disc.These two pieces are joinedtogether with an epoxy bond, thus forming a silicon load cylinder.The piezo-resistive elements are effectively embedded in the siliconi load cylinder,andgxmreBssive loads may be read ily applied to the piezoresistive elementsin a uniform munr without undesirable stress ccntrations by the application of a nrmanil load.A silicon disc 0.76 am thick by about 0.44 cm in diameter is employed asa support disc.This second piece of silicon, having approximately thesame overall dimensions but containing two smallI through-holes. for leadexits, is joined to the sensor disc with a thin epoxy layer.It is clear that a silicon load cylinder loaded uniaxiafly may beconsidered as a "load call."The approachiThis suggests an attractive technical solution.malnntdkes use of the transverse piezoresistiveproperties of silicon to comp ressive load.Wmte the load cylinder isappropriately mounted in a transducer structure, an applied pressure resultsin a compressive load on the load disc; this in turn results in oczressivestress in the load disc.Since the piezoresistive elemients are in the planeof the load disc, a normal compressive stress is exerted on these elements.If an appropriate crystallogrsiic orientation is chosen, a significantresistance change is observed in the active piezoresistive elements due tothe transverse piezoresistive effect.Father than a clamped-edge deflect-ing disc, the structure is an integrated sensor load-disc wafer bonde toa solid silicon support plate and loaded in uniaxial cx Freassion.17

'he theory of such a device was discussed in a 1968 paper byDr. Anthoy Kurtz and Charles Gravel entitled, "Semiccxutor TransducersUsing Transverse and Shear Piezoresistance", which is listed in thebi liography.The pressure sensor makes use only of the transverse piezoresistivecoefficient.If a cylinder of silicon containing properly orientedeedpiezoresistive stress sensing elements is subjected to uniaxial compression,an output proportional to body stress is obtained. This configuration shownin Figure 3 uses two arms of a Wheatstone bridge having a large negativetransverse stress gage factor as active bridge elements and tw arms havingessentially zero gage factor as inactive or dummy bridge elements.A suit-able orientation for p-type silicon to achieve this is the plane of disc(110), direction of active elements (110) and direction of dummy or inactiveelements (100).The sensitivity of such a device used in a Wheatstombridge configuration is given byAvV12te(10)twhereGPt is the effective transverse gage factore is the strainFor the orientation and doping levels chosen -50So for an output of 30 nv/JV, the required stress can be calculated asfollows:18

3qAV1V-vThusrss(13(-50)12AVAao5(12)E-Ex g50V (14)-248.2 MPa(15)ve stresses of the order of 248.2 MPa on the loadcylinder are required.Such a stress level is well within the cmpressivestrength of silicon, which is on the order of several thousand iPa.Frequency response for such a device is inherently high.The frequencyresponse of the silicon load cylinder is given byfn2 f(16)2rK rR2 E(17)m rR2 pT(18)whereR is the radius of the discE is Young's modulus - 1.93 x l05 MPaT is the thickness of the discp is the density of silicon 2.3 g /Cu Thurefore1,.47 x105 Hzfn19-A jj-.*."(19)

With thicknesses of 0.51 mm to 0.75 mm, the theoretical natural frequencyis 2 to 3 MIz.Two sensor configurations ware developed and fabricated during thisThe initial design uses an integrated sensor load disc typeeffort.SQ33-350-163.A computer generated diffusion program is presented inTable 1 and a mask drawing of the device is shown in Figure 4.The programof Table 1 calculates relevant performance parameters and appropriatediffusion times for elements of various diffused line widths.is based on the classical diffusion equations.The programInitial measurements ontransducers fabricated with this sensor indicated that the contract sensitivity goals could not be met.A redesign of the sensor to achieve a higher gage factor and a bridgeresistance increase from 350 to 750 9 was ac4cxplished as part of thiseffort.The higher gage resistance allowed the excitation level to beincreased from 5 V to 10 V. The new configuration, which uses an integrated sensor load disc type C22-750-175, was a very successful designwhich allowed the comtract sensitivity goals to be met.See Table 2 andFigure 5.b.Device Fabrication-For proper stress transmission, the trans-versely active piezoresistive elemnts umst essentially be embedded in thesilion load cylinder.two-piece structure.This is accplished by making the load sensor aThe first piece is a disc containing the piezoresis-tive element, and the second piece is a silicon support disc rigidly bondedto the first by the use of an epoxy adhesive.*20V.-,

. F.;U;tnU; WU; U U00000000000000co'-tI *-.r.r.r.*li.

.10 FtTYP* .10Jo1.97ALL DIMENSIONSFIGURE 4. SC33-350-175 DIFFUSION MASK22

44HHH4444Ooc0coGH-H-0c; ; a C;CDa000000HHHHIHrr--;CHLnLr L)LLLALnI-r---a a000D00D0Ca0000C)00)(DI-D-Ln Chel0C(:4 04RI) 0 (D a-O C'inLflNLwN Co N023NNNN M00000M0-

/.10 R. TYP/LIEWDT6q1.97.2ALL DIMENSIONSARE IN m.FIGURE 5. Cll-350-175 DIFFUSION MASK24

The load sensor disc is fabricated using standard photolithographictechniques to obtain the diffused pattern.The approximate depth of such adiffused layer is about 1 m.4.MECACAL DMIGNThe basis for the mechanical design of the transducer is theHKS-1-375.This transducer has proven rugged and reliable through manyyears of production at Kulite and field test use by the Air Force Weaponslaboratory and other agencies.The evaluation of this design has been wiiedocurented and will not be repeated here.Figure 6 shows the mechanical construction of the HKS-II-375.The con-struction is essentially monolithic with no moving parts, allowingextrenely high shock acceleration hardness; the measurand is coupled to thesilicon disc through a small port in the sensing face of the unit via thethermal barrier material, TBS-758.This transducer employs an integratedsensor silicon disc using a conventional clanped-edge diaphragm.Thetechniques of construction were chosen and developed to meet the requirementsof survivability and performance in the high explosive test envirorment.Preliminary testing of the load cylinder was done in this configuration.two-sided clamping arrangement is accomplished when the load cylinder inFigure 3 is employed with the HKS-11-375 hardware of Figure 6.The loadcylinder is clamped between the transducer body and the subassembly housing.It became apparent, however, that this mechanical configuration wasinappropriate for this type of sensor.Test data given in a subsequentsection show that this dual-sided clamping configuration interferes withthe proper response of the silicon load cylinder.25A

THERMAL BARRIER RETAINERPORTED STEEL CAP ELECTRONBEAM WELDED TO SENSING ENDOF TRANSDUCERTHERMAL BARRIER (GE TBS 758)PHOTON BARRIERELEMENT/ TRANSDUCTION(INTEGRATED SENSOR SILICON DISC)LEAD TRANSITIONREFERENCE CAVITYSUBASSEMBLY HOUSINGCARRIER RETAINER21. 8mREFERENCE TUBECOPPER SEAL WASHER"'PIGTAIL CONDUCTORS (4)BRAIDSHIELDTRANSDUCER BODY(7/16 HEX. 3/8-24 THD.)EPOXY FILL (TOTAL)PIGTAIL LEAD -4 COND.(POLYURETHANE JACKET)FIGURE 6.HKS-11-375-1OK BLAST PRESSURE TRANSDUCER26:.".:./,;

The configuraticn shawn in Figure 7 was investigated; several aspectsof this design are notewrthy. The silicon load disc is mounted to a thidcsilicon support disc to form a silicon load cylinder.Thisechniqueinsures a perfect thermal match betweean the sensor and its mounting. Thisis quite important for thermal and mechanical stability.The silicon loadcylinder is eoxy nmunted to a two-piece metal sipport housing to form a loadsensor subassembly.Th sensor subassembly is electron ben welded to thetransducer threaded body.This technique insures the mechanical stabilityof the transducer. Proven techniques of construction are eployed, includingthe leadwire interconnection systeu used in the HKS-1-375 and the combinaticn of the GE TBS-758 thermal barrier and aluminum photon barrier employedin that transducer.The aluminum photon barrier precludes undesirable flashthermal responses.Measurements shwd that the configuration of Version I did not responto static pressure. Essential to the operation of the device is the couplingof the measurand uniaxially to the sensor. The structure of Figure 7allows the applied pressure to distribute itself hydrostatically through thelow modulus TBS interface layer.An analysis sham that all crystallographicorientations for P-type silicon are insensitive to hydrostatic pressures.In fact, the configuration responded well to transient blast pressure wave*fronts but could not be statically calibrated.The ability to effect matci-ing static and dynamic calibration is a major advantage of the piezoresistiveapproach.The structure of the HEC-1-375 Version II, shown in Figure 8, was developed to address this lack of static response by the use of an alternativecoupling technique.27*11I'-.J ,:',i ",",:i '';;". '''"], " ,, :' '

SILICON SUPPORT DISC.,,,-THERMALBARRIER RETAINERSILICON LOAD DISCSILICON SEMBLY HOUSINGSEALING 0-RING-TRANSDUCERBODY(7/16 HEX, 3/8-24 THD.)POLYURETHANE PIGTAILFIGURE 7HLC-1-375-50K VERSION I LOAD CYLINDER TYPEBLAST PRESSURE TRANSDUCER28

BOSSED PRESSURE COUPLERLOAD SENSORSUBASSEMBLYSUBASSEMBLY HOUSING--E.B. WELD--- EPOXYFIGURE 8. HLC-1-375-50K VERSION 11 LOAD CYLINDER TYPEPRESSURE TRANSDUCER EMPLOYING BOSSED PRESSURE COUPLER29--,z'.-.

The transduc er of Figure 8 has a bossed natal force coupler toserve as force collector.72is force coupler, wh ich is integrally machineddandl electron bam welded, serves to couple the pressure nassurand to thesilicon load cylinder without need for a port.The metal face sere asan excellnt.photon barrier and supplies ernough thermal and mechanicalprotection to the silicon load cylinder so that the lTheral Barrier Siliconlayer znrmlly euployed is not required.If the 116 is found to be desir-able for any reason, it may be added to the structure in the usual manner.jAn additional benefit of the metal face is that the relative dimsmions ofthe boss and the groove nmy be varied to adjust the transducer sensitivityand range. cprierialdata on the subjectx 9s vr have verified the per-forinance of this design.304-1S

IV.1.PRGAPROGRAI;EJLTS ANDDTSMAThe following was accomplished during the course of the cotract:a.A number of transducers employing SQ-33-350-175 load cylinderswere fabricated.b.Preliminary masurents were made on transducers fabricated withHKS-11-375 parts.The SQ-33-350-175 load cylinder did not possess sufficientsensitivity to meet the contract; and the HKS-11-375 construction was notsuitable for use with the load disc.C.Accrdingly, redesigns of the load sensor disc and the transducerstructure were initiated.A new sensor mask (C12-750-175) and a load sensordisc of a higher gage sensitivity and bridge resistance ware fabricated.The HIC-1-375 transducer design was accomplished and a number of transducerassemblies ware manufactured.This design was unsuitable; it respondedproperly in a dynamic mode but was nonresponsive in a static rode.d.The final design emerged as the hlC-I-375 Version II.design was quite successful.Two versions were fabricated:Thisa long versionwith a relatively thick boss and a short version with a thinner boss.Itwas felt that the stiffer, thicker boss might be necessary even at thepenalty of higher mass loading.In fact, both versions performed withinthe contract goals.e.Five transducers of Version II were fabricated and sIoxk tubetested at Kirtland.31?t 1.R0

2.TT ESUTSAn initial evaluation was performed with standard HKS-11-375 hardwareand an S033-375-175 load disc.Thsensitivity of several sensors with 5 VDC excitaticn islistedbelow:TABLE 3. SENSITIVITY CF SQ33-375-175 LOAD CMCUnit No.Sensitivity W/Pa)114.5627.5726.2The above represents an average full-scale output of 165 zW.This issubstantially below the contract goal of 300 WV nominal.Drring the testing, the units displayed substantial hysteresis (apprm.5%

KILOBAR BLAST PRESSURE TRANSDUCER-3.4 KILOBAR PIEZORESISTIVE BLAST PRESSURE 00 SENSOR USING SILICON LOAD CELL TECHNIQUE Joseph R. Mallon, Jr Kulite Semiconductor Products, Inc. 1039 Hoyt Avenue Ridgefield, NJ 07657 June 1982 DTIC ELECTE Final Repo